1. Field of the Art
This invention relates generally to interferometers which are used for inspecting surface conditions or surface configurations of optical lenses or other precision optical elements by way of interference fringes, and more particularly to an alignment verification system to be used with an interferometer for adjusting a test piece under interferometric inspection into an aligned state with respect to the optical axis of a reference member on the part of the interferometer.
2. Prior Art
For inspecting machining accuracy, for example, of an optical precision element (hereinafter referred to simply as "test piece" for brevity) by the use of an interferometer, it is the usual practice for the interferometer to employ a laser light source, diverging the light rays from the laser light source into a flux of a predetermined diameter through a diverging lens and passing the light flux through a collimator lens to form parallel light rays to be shed on a reference member with a model or prototype surface (hereinafter referred to as "a master surface") for the test piece to be inspected. The parallel light rays are partly reflected on and partly transmitted through the reference member. The transmitted light rays from the reference member are reflected on the inspecting surface of a test piece to go back together with the light reflection from the reference member, along a path which is turned through 90.degree. toward an observation means where the wave surfaces of the two light reflections are superposed one on the other to observe the interference fringes resulting from interference of the two superposed wave faces, assessing the surface condition of the test piece under inspection by way of the number and shape of the interference fringes.
In this connection, the light path from the laser light source to the reference member as well as the light path from the reference member to an image pickup means, which serves to form an image of interference fringes, is built into a housing of the interferometer with the components of the respective light paths preadjusted in precisely aligned state. However, each one of the test pieces, which need to be replaced one after another for successive inspection, is normally set in position on a support means outside the interferometer housing. Therefore, each time a test piece is set on the outer support means, there arises a necessity for fine adjustments for bringing the optical axis of the test piece on the support means exactly into alignment with that of the reference member on the side of the interferometer. In this regard, it has been the usual practice to mount the support means for the test piece on an adjustment stage which is capable of three-dimensional adjustments, namely, capable of adjusting the position of the support means in horizontal directions as well as in tilting directions.
Nevertheless, there is no guarantee that a test piece is set strictly in a predetermined position or posture on the support means no matter whether it is set by a manual operation or automatically by the use of a pick and place means. For instance, a positioning mechanism to be provided in association with the outer support means is normally arranged and operated on the basis of the outer shape of the test piece, and invariably needs adjustments depending upon the particular shape of the test piece to be inspected. Precise positioning of test pieces based on their outer shape becomes difficult especially in case they have a relatively large dimensional tolerance in outer configuration. Besides, accurate inspection and measurement are rendered impossible if a test piece is in a misaligned state relative to the reference member on the side of the interferometer. Therefore, the support means is usually provided with an alignment system thereby to bring each test piece on the support means into alignment with the reference member. A typical alignment system includes a bi-axial adjustment mechanism for tilting the support means in the directions of perpendicularly intersecting X- and Y-axes together with a test piece, which is set on the support means, in combination with an alignment verification means including a conversion lens to converge light reflections from the reference member and test piece into spot images of a predetermined diameter on a predetermined plane. The test piece on the support means is adjusted into an aligned position relative to the reference member by operating the bi-axial adjustment mechanism in the X- and Y-axis directions in such a way that the spot image of the reflected light from the test piece is superposed exactly on the spot image of the reflected light from the reference member.
In bringing the optical axis of a test piece into alignment with that of the reference member by way of the two spot images, the bi-axial adjustment mechanism needs to be operated according to detected positional deviations between the two spot images. In this regard, it has been the conventional practice in the art to employ a TV camera for capturing the test piece spot image and the reference spot image, detecting the position of the test piece spot image by means of a coordinate detection system having its origin at the position of the reference spot image, and operating the bi-axial adjustment mechanism according to the detected positional signal in a direction of zeroizing the positional deviation of the test piece spot image from the reference spot image. What is required here is to operate an adjustment mechanism in such a way that the test piece spot image is shifted toward and superposed on the reference spot image which is at a fixed position. The adjustments of this sort, however, can be attained without resorting to a costly and complicate detection system using a TV camera in combination with a coordinate detector.